Blue printing



Patented Apr. 23, 1935 UNITED STATES PATENT OFFICE BLUE PRINTING No Drawing. Application September 26, 1933, Serial No. 691,037

7 Claims.

This invention relates to the art of blue printing and more particularly to a method of developing exposed blue prints.

Blue printing is a process of reproducing drawings, prints and the like, the reproduction having a blue background with white lines thereon. In the blue printing process a drawing or the like is made on a translucent paper or tracing cloth, and this is laid over a sheet of sensitized blue printing paper and exposed to the light in such manner that the light passes through the transfer paper onto the sensitized paper. After exposure to light the sensitized paper is washed in water, treated with a developing solution, again washed in water and finally dried. Ordinarily the sensitizing material with which the paper is coated or impregnated consists of a mixture of potassium ferric oxalate, potassium ferricyanide and potassium oxalate. When the sensitized paper is exposed to light the potassium ferric oxalate is reduced to form ferrous oxalate and the potassium ferricyanide is reduced to potassium ferrocyanide; when the exposed paper is washed with water these products react to form ferrous ferrocyanide andthe oxalate is largely removed. On treatment with the developing agent, the ferrous ferrocyanide is oxidized to form the insoluble blue ferric ferrocyanide.

An alkali metal dichromate, specifically potassium dichromate, is commonly used as the developing agent in blue printing. After washing, the exposed print is immersed in or washed with a stream of the potassium dichromate solution to develop the blue color. Although this developing agent has been used for many years and in general produces satisfactory prints, it nevertheless has certain disadvantages. It causes stains if contacted with the skin or clothing of the operator and also causes brown stains on the blue print if the print is not thoroughly washed with water after the developing treatment. Also, its contact with the skin often causes dermatitis, or inflammation of the skin. Dichromate development also often causes an undesirable dark purplish color on the blue print, especially if the print has been overexposed. This purplish color is often referred to as a burnt appearance.

Furthermore as the dichromate developing solution is used, it is gradually decomposed to form an insoluble chromic oxide sludge which precipitates in the developing tank or apparatus.

An object of this invention is to provide an improved process for developing blue prints. A further object is to provide a blue print developer which is free from the disadvantages of the dichromate developer. Other objects will be hereinafter apparent.

These objects are attained by developing exposed blue prints by treatment with a solution of urea peroxide. I have discovered that urea peroxide solution gives somewhat better results than a dichromate solution in the development of blue prints and is free from the disadvantages inherent in the use of dichromate. In general, development with urea peroxide solution results in prints which have a brighter shade of blue and a better contrast between the blue and the white. A blue print developed with urea peroxide has no discoloration or burnt appearance, even though the print has been over-exposed. The urea peroxide developing solution does not stain paper, clothing or 1 skin and is not injurious to the skin. Residual urea peroxide has no deleterious effect on the print, therefore the usual final washing step in the developing process may be omitted if desired. There is no formation of insoluble material in the urea peroxide developing solution, even after long periods of use.

In carrying out my invention the blue print is exposed to light in the usual manner, using the same exposure time as for dichromate development, washed with water in the usual manner and then is developed by suitably treating it with a urea peroxide solution, for example by immersing the print therein until the desired shade of blue is attained. The concentration of the urea peroxide solution may vary between wide limits and may be as low as 0.04 gram ofurea peroxide per liter of solution. For work where it is desired to develop the print within 3 to 5 seconds, I prefer to use a solution containing around 0.3 gram per liter of urea peroxide. If desired, higher concentrations may be used;- satisfactory results may be obtained even with a saturated solution. However, there is ordinarily no advantage in using a solution stronger than about 0.3 gram per liter. After the print has been developed to the desired extent, it is removed from the urea peroxide solution and then may be dried without further treatment. If desired, the developed print may be washed in the usual manner before drying; however this washing is not essential to secure good results.

Obviously, the urea peroxide solution may be.

used in any suitable manner, that is, it may be applied by the various method that have heretofore been used for the dichromate developing solution. During continued operation, the urea peroxide gradually loses its developing power. For example, a solution containing around 0.3

developing power.

gram per liter, which is capable of developing the ordinary blue print in from 3 to 5 seconds, usually will maintain this developing power for from 2 to 5 days depending upon the number of prints developed. Loss of developing strength will 'become apparent to the operator when it is observed that the solution requires a longer time of treatment to bring out the desired depth of color. When such decrease in developing power is observed the solution preferably is replenished by the addition of a suitable amount of urea peroxide. Preferably, replenishment is made with solid urea peroxide, a relatively stable, crystalline substance, usually marketed in a powdered form, which is easily transported and handled. The urea peroxide. very quickly dissolves in water; hence there is no interruption of the developing process when replenishment is made. If desired, a strong aqueous solution of urea peroxide may be made up and this used to replenish the developing bath. However, I prefer to replenish with the solid, anhydrous urea peroxide; since the aqueous solution is relatively unstable and gradually decomposes to form a solution of urea.

After an extended period of use under ordinary conditions, it will be found that urea peroxide begins to lose its developing power at a more rapid rate than'in the case of a freshly prepared solution. This is probably caused by the presence of iron salts which are dissolved from the blue print paper and which catalytically accelerate the decomposition of the urea peroxide. For this reason, I prefer to replace the solution with a freshly prepared urea peroxide solution from time to time as the concentration of iron salts becomes high, as observed by a rapid decrease in In ordinary practice, this will require replacement by a freshly prepared solution about once each Week.

Certain heavy metals commonly used forconstruc ting blue print developing apparatus, e. g. iron, copper or brass, have a certain catalytic effect, causing the decomposition of the urea peroxide. To prevent this catalytic eifect, I prefer to use developing apparatus made of or lined with a material which has little or no catalytic action on the decomposition of urea peroxide, for example, ceramic materials, tin. 18-8 chrome-nickel alloy steel, aluminum, or peroxideresistant rubber. A satisfactory non-catalytic surface also can be produced'on the usual iron or steel equipment by applying thereto a coating of asphalt, sodium silicate or other inert, protective coatings". An asphalt coating is conveniently applied by painting the metal surface with a solution of asphalt in benzene or other volatile solvent. A silicate coating may be applied to an iron surface by application of a hot solution of sodium silicate, followed by drying.

From the foregoing, it is apparent that my urea peroxide developer has many advantages over, the dichromate developer heretofore used. The urea peroxide developing solution causes no stains or discolorations on the blue prints or the operators skin or clothing and has no injurious effect on the skin. It does not precipitate insoluble material and it produces a better colorand better contrast between blue and white in the developed print than is ordinarily attained with the dichromate developer. A further advantage is that the step of washing after developing may be omitted, since the residual urea peroxide or urea has no deleterious effect on the print. The omission of the final washing results in saving considerable time when large quantities of prints are being produced.

I claim:

1. A process for developing an exposed blue print in which the blue coloring is imparted to the print by the oxidation of ferrous ferrocyanide into the insoluble blue ferric ferrocyanide comprising contacting said print with a solution of urea peroxide.

2. A process for developing an exposed blue print in which the blue coloring is imparted to the print by the oxidation of ferrous ferrocyanide into the insoluble blue ferric ferrocyanide comprising contacting said print with a solution of urea peroxide containing not less than about 0.04 gram of urea peroxide per liter.

3. A process for developing an exposed blue print in which the blue coloring is imparted to the print by the oxidation of ferrous ferrocyanide into the insoluble blue ferric ferrocyanide comprising contacting said print with a solution of urea peroxide containing around 0.3 gram of urea per xide per liter.

4. A process for developing an exposed blue print in which the blue coloring is imparted to the print by the oxidation of'ferrous ferrocyanide into the insoluble blue ferric ferrocyanide comprising contacting said print with a solution of urea peroxide and drying the developed print without further treatment.

5. A process for developing an exposed blue print in which the blue coloring is'imparted to the print by the oxidation of ferrous ferrocyanide into the insoluble blue ferric ferrocyanide comprising contacting said' print with a solution of urea peroxide containing not less than about 0.04 gram of urea peroxide per liter and drying the developed print without further treatment.

6. A process for developing an exposed blue print in which the blue coloring is imparted to the print by the oxidation of ferrous ferrocyanide into the insoluble blue ferric ferrocyanide comprising contacting said print with a solution of urea peroxide containing around 0.3 gram of urea peroxide per liter and drying the developed print without further treatment.

7. A blue printing process adapted for preparing blue prints in which the blue color is imparted to the print by oxidation of ferrous ferrocyanide into the insoluble blue ferric ferrocyanide, comprising exposing blue print paper to light, washing the exposed print with water, immersing the exposed and washed print in a solution of urea peroxide containing around 0.3 gram of urea peroxide per liter until the desired color is developed and then drying the developed print without further treatment.

JOSEPH S. REICHERT. 

